A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

learn more… | top users | synonyms

1
vote
1answer
221 views

Can a super-extremal charged black hole be made out of electrons only?

In a previous Question it was argued that it would be impossible to add enough charge to a black hole to make it pass the extremal black hole limit since adding charge would increase the mass of the ...
11
votes
5answers
863 views

Double slit experiment near event horizon

What happens if you perform a double slit experiment near an event horizon, if one of the slits is outside, one is inside the event horizon?
4
votes
1answer
479 views

How does an object falling into a plain Schwarschild black hole appear from near the black hole?

I know that when viewed from infinity (or from a very large distance from the black hole event horizon), an object that falls into the black hole will appear to slow down and will become more and more ...
2
votes
1answer
596 views

Schwarzschild black hole geometry in Novikov coordinates

As I am a layman in gravitation I would really appreciate any suggestions on how to approach and tackle the following problem: Choose a time-symmetric 3-geometry for the initial geometry of ...
1
vote
1answer
360 views

Charges and Topology

I must apologize, I was a little bit excited when I began understanding some of this, I can not say I can compete with professionals, and words are still difficult concepts. In (1) S.H. et al. it is ...
2
votes
2answers
119 views

Moving “virtual” singularity?

Imagine two close, really big black holes rapidly spinning around each other. That setup would emit a terrible amount of gravitational waves. My question is, could those gravitational waves, if big ...
3
votes
2answers
405 views

Singularities and quantized space time

Discrete space time quanta would solve the problems of infinite densities for singularities in General Relativity and Quantum Gravity by imposing a non zero limit on the minimum radius of black holes. ...
1
vote
2answers
241 views

Primordial Black Hole Detection

There's ample direct evidence for the existence of galactic and stellar mass black holes. However, there is no such direct evidence of primordial black holes, those formed after the Big Bang. A recent ...
5
votes
2answers
439 views

Can a black hole be formed by radiation?

I'm trying to find out if black holes could be created by focusing enough light into a small enough volume. So far I have found (any or all may be incorrect): Maxwell's equations are linear, ...
4
votes
2answers
935 views

What role does electrical charge play in black holes?

Not having studied General Relativity, I have sometimes been puzzled by references to the behaviour for "classic" black holes — as they are popularly portrayed — as being true for black ...
11
votes
4answers
576 views

Big Bang snuffed by a black hole?

Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what ...
9
votes
1answer
658 views

Second Law of Black Hole Thermodynamics

I've been looking for a satisfying proof of this, and can't quite find it. I read the brief proof of the black hole area theorem in Wald, which is similar, but doesn't quite come down to the actual ...
4
votes
3answers
110 views

What would a rotating black-hole look like to a “geo-stationary” observer orbiting the black hole

A rotating black hole is believed to contain a ring singularity rather than a point. However, if an astronaut is orbiting the black hole at exactly the same angular velocity as the blackhole (in ...
2
votes
1answer
25 views

Would the time dilation from being in low orbit around a black hole delay/slow the effects of quantum tunneling?

If quantum tunneling will cause rigid objects like rocks to rearrange their atoms into a slow-moving liquid-like state in ~10^65 years, could an object delay this fate through time dilation in a low, ...
23
votes
6answers
3k views

How fast a (relatively) small black hole will consume the Earth?

This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth". Consider a small black hole, that is somehow got inside the Earth. Under ...
5
votes
2answers
226 views

Hayden-Preskill informational mirrors and decryption

I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long ...
7
votes
1answer
303 views

Charged particle close to a charged black hole - what happens?

Let's assume the Reissner–Nordström metric (charged black hole, non-rotating), for simplicity. The black hole is charged with a powerful electric charge. There's a particle nearby, of non-zero mass, ...
4
votes
2answers
586 views

What will the universe look like for anyone falling into a black hole?

I've heard that, from the perspective of an external observer, something falling into a black hole will eventually look "frozen": light waves will move to the infrared and further into lower ...
2
votes
2answers
110 views

Returning Light

What happen if someone is just close enough to a black hole, and he emits light tangentially out, and the light got bent and circles around, what will he see, can he see the light source from his ...
8
votes
1answer
484 views

How do we know that black holes evaporate?

This has been bugging me for some time. As I understand it, Hawking radiation is the result of the mismatch between the vacuum state of a quantum field as seen by a free falling observer (falling ...
2
votes
1answer
129 views

Notational(?) Question in Whiting's Paper “Mode Stability of the Kerr Black Hole”

I am a math grad student attempting to read Bernard Whiting's paper "Mode Stability of the Kerr Black Hole." If you are in a university network, the paper should be easily found by a google search. At ...
8
votes
3answers
288 views

Collision of charged black holes

Suppose there are two charged black holes which collide to form a bigger black hole. But when they combine, a lot of potential energy of the system is lost/gained depending on their charges (the ...
5
votes
1answer
516 views

Why isn't black hole information loss this easy (am I missing something basic)?

Ok, so on Science channel was a special about Hawking/Susskind debating black holes, which can somehow remove information from the universe. A) In stars, fusion converts 4 hydrogen into 1 helium, ...
6
votes
2answers
278 views

How can one reconcile the temperature of a black hole with asymptotic flatness?

A stationary observer very close to the horizon of a black hole is immersed in a thermal bath of temperature that diverges as the horizon is approached. $$T^{-1} = 4\pi \sqrt{2M(r-2M)}$$ The ...
2
votes
5answers
486 views

Black Hole Singularities

If two black holes collide and then evaporate, do they leave behind two naked sigularities ore? If there are two, can we know how they interact?
2
votes
3answers
217 views

What is the strange event in this simulation of a galactic collision?

I was watching this video on YouTube: 2 Spiral Galaxies w/Supermassive Black Holes Collide Around half way, and again almost at the end, the black holes seem to suddenly give off some sort of force ...
1
vote
1answer
612 views

Has Cosmological Natural Selection been disproved?

I've been reading Lee Smolin's Life of the Cosmos. Great book and it makes a lot of sense that the conditions in black holes are the same as conditions at the big bang. Question is, has his theory ...
6
votes
1answer
753 views

On black holes, Hawking radiation and gravitational atoms

Over the past hour or so I've been following one of my standard physics-based, wanders-through-the-internet. Specifically, I began by reviewing some details of dark energy theory but soon found myself ...
3
votes
1answer
173 views

Please justify invoking logical positivism to causal patches and black hole interiors in quantum gravity!

Logical positivism is often invoked to explain why external observers can't talk about black hole interiors, or why we can't talk about what happens outside our causal patch in inflationary models. ...
1
vote
2answers
261 views

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle?

Can the equivalence principle be tested to high precision in a human-sized lab falling through the horizon of a black hole, in principle? By "human-sized lab" I mean a lab the size of the ...
4
votes
2answers
239 views

Do all of our discoveries of black holes in nature depend on the validity of GR?

In the question Is there a black hole in the centre of the Milky Way? the answer by Motl seems to all but say the existence of that black hole is a fact (see also Evidence for black hole event ...
6
votes
1answer
774 views

Time dilation when falling into black hole

I know that if one astronaut falls into a black hole, then a distant observer will see him take an infinite amount of time to reach the event horizon (provided the observer can see light of ...
0
votes
2answers
343 views

Black Holes and Energy Conservation

A supernova's Ni/Fe core remnant with mass exceeding about three solar masses would result in a black hole. Although the core's temperature was in the ~1E10K range, the black hole temperature based on ...
3
votes
2answers
3k views

Strongest force in nature

Possible Duplicate: What does it mean to say “Gravity is the weakest of the forces”? It is said nuclear force is the strongest force in nature.. But it is not true near a black ...
4
votes
2answers
224 views

Can a trapped surface be formed by a mass configuration outside of that trapped surface?

Can a trapped surface be formed without any massive bodies inside that trapped surface, but only by a configuration of massive bodies surrounding the trapped volume?
4
votes
2answers
2k views

How long does it take a black hole to eat a star?

I presume the answer is that it depends on the mass and size of the star and black hole, but I was wondering if somebody could provide some rough bounds (e.g. hours vs thousands of years). By ...
2
votes
5answers
974 views

Is there a black hole in the centre of the Milky Way?

Is it true that the whole galaxy is actually revolving, and powered by a black hole? Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the ...
2
votes
2answers
460 views

Are black holes really that special?

Science and science fiction alikeScience fiction describes black holes as these amazingly different entities in space that don't behave according to the same laws of physics that the rest of the ...
6
votes
3answers
341 views

How can you tell if a critical energy density is actually a black hole?

Here's a question inspired by Edward's answer to this question. It's my understanding that the average energy density of a black hole in its rest frame is $\rho_\text{BH}(A)$, a function of surface ...
11
votes
3answers
586 views

Evidence for black hole event horizons

I know that there's a lot of evidence for extremely compact bodies. But is there any observation from which we can infer the existence of an actual horizon? Even if we are able to someday resolve ...
4
votes
3answers
316 views

Information about an expanding event horizon

Assume an observer sent a beam of photons close to an event horizon, say at some distance x (a distance far enough to avoid the photons falling in.) This light would still be observable, albeit red ...
13
votes
5answers
1k views

Are elementary particles ultimate fate of black holes?

From the "no hair theorem" we know that black holes have only 3 characteristic external observables, mass, electric charge and angular momentum (except the possible exceptions in the higher ...
6
votes
3answers
586 views

Black holes in a head-on collision

Assume two uncharged non-rotating black holes traveling straight at each other with no outside forces acting on the system. What is thought to happen to the kinetic energy of these two masses when ...
3
votes
1answer
556 views

What is a maximal analytic extension?

Can someone explain (as rigorously as possible) what is involved in analytically continuing, say, the Schwarzschild solution to the Kruskal manifold? I understand the two metrics separately but I'm ...
4
votes
3answers
637 views

Do singularities have a “real” as opposed to mathematical or idealized existence?

I was thinking of, for example a Schwarzchild metric at r=0, i.e. the gravitational singularity, a point of infinite density. I realise that there are different types of singularities--timelike, ...
1
vote
5answers
264 views

High-spin neutron star

Suppose a large neutron star were to be spun-up by a particular pattern of mass-accretion. The increased centrifugal force could presumably mitigate the increased gravity thus delaying gravitational ...
12
votes
3answers
2k views

If the universe were compressed into a super massive black hole, how big would it be?

I understand only a little of general relativity, but that's why I'm here! :) Consider the hypothetical situation of some extra-terrestrial intelligence pushing all the mass in the universe, every ...
2
votes
3answers
262 views

What is the color shift if light passes between two (or more) black holes?

If we observe light that originates X light years away, but it passes between black holes X/2 light years away, will it be normal or red shifted or blue shifted? What if the black holes were X/4 or ...
5
votes
1answer
213 views

What kind of systems of black holes satisfy the laws of black hole thermodynamics?

I've come across black holes thermodynamics multiple times recently (both at this site and elsewhere) and some things started bugging me. For one thing, first law bothers me a little. It is a ...